In desert plants, transpiration rates decreased before photosynthetic rates when plants were entering a period of water stress. This may have adaptive consequences. A difference of -5 bars in the soil-moisture potential had considerable importance in reducing the rate of transpiration. In Helianthus annuus L. (sunflower) the photosynthetic rate decreased before the transpiration rate in contrast to Great Basin-Mojave Desert plants, and the changes occurred with a -1 bar difference in soil-moisture potential. Morphological changes in three desert plant species [Artemisia tridentata Nutt., Ambrosia dumosa (Gray) Payne, Larrea tridentata (Ses. Moc. ex DC) Cov.] as the soil-moisture potential decreased are given. With a mesic species, H. annuus, 20 percent reduction in photosynthesis and transpiration was reached at higher soil-moisture potentials than with the desert plants. Loss of net photosynthesis occurred in A. dumosa (a summer deciduous shrub) as Ψ soil reached -48 bars in the field, whereas L. tridentata (an evergreen shrub) at the same time was able to maintain a water potential difference between soil and plant of -10 to -15 bars and continue net CO2 gain well into the summer months.
Clark, S. B.; Letey, J. Jr.; Lunt, O. R.; Wallace, A.; Kleinkopf, G. E.; and Romney, E. M.
"Transpiration and CO2 fixation of selected desert shrubs as related to soil-water potential,"
Great Basin Naturalist Memoirs: Vol. 4
, Article 15.
Available at: http://scholarsarchive.byu.edu/gbnm/vol4/iss1/15